In the treatment of various orthopedic and spinal ailments and defects, it is desirable to stabilize or partially or wholly immobilize two or more bony segments, with bone arthrodesis or fusion being the desired outcome. By applying compressive force across the site, bone growth is enhanced according to Wolff's law. This is believed to decrease healing time and increase fusion quality.
Similarly, in the treatment of fractures and other orthopedic conditions, stabilizing or immobilizing devices are often placed on bone fragments to maintain bony alignment and impart stability to promote healing. Healing can be further promoted by creating compression across the fracture site intra-operatively and, ideally, allowing dynamic compression across the fracture site post-operatively.
Specifically, spinal fusion is one example of a surgical procedure that is used to stabilize or immobilize adjacent vertebrae in the treatment of an injury or degenerative condition. During the procedure, the intervertebral disc is removed and the intervertebral space is filled with bone graft material and/or a fusion cage. A bone plate is typically used to provide stability to the affected spinal segment, keeping the bone graft material and/or fusion cage in place and providing rigidity.
Often, however, the bone graft material or the bony fragments exhibit bone resorption, which is the process of osteoclasts breaking down the bone and releasing minerals into the bloodstream. As the bone graft material resorbs, there is a loss of contact with the host bone and less compression of the bone graft material, leading to progressively less likelihood of incorporation and healing of the fusion. This process is usually the result of a lack of stimulus for bone maintenance, i.e. compression. This also leads to an exponentially increased load on the bone plate, resulting in an increased occurrence of failed implants, which typically requires revision surgery and means longer recovery times for patients.
The use of a bone plate that allows for dynamic compression post-operatively would enhance the bone arthrodesis process. Dynamic compression would stimulate the healing of the bone graft material to the host bone, resulting in a more rapid and solid fusion. In the case where the bone graft material undergoes resorption or a fusion cage is compromised, dynamic compression would allow for the vertebral column to shift axially, thus promoting the maintenance of bony contact and compression stimulus at the arthrodesis site. In a worst case where the bone graft material completely resorbs and adjacent host bones are then touching, compression stimulus is still applied and arthrodesis can still take place, thereby imparting spinal stability.